Proton therapy is a type of external beam radiation therapy that is characterized by the use of a beam of protons to irradiate diseased tissue. A chief advantage of proton therapy over other conventional therapies such as X-ray or neutron radiation therapies is that proton radiation has the ability to stop in matter—treatment dosages are applied as a sequence of proton beams with several energies three-dimensionally. The dose deposition of each monoenergetic, thin (“pencil”) proton beam in a medium is characterized by a sharp increase in dose deposition (Single Bragg Peak) directly before the end of the proton range (i.e. beam depth), and thereby limiting the inadvertent exposure of non-target cells to potentially harmful radiation. The pencil beam scanning technique allows the deflection of monoenergetic proton beams to prescribed voxels (in transversal direction/x- and y-coordinates for associated beam depths) in medium—the so called spot scanning technique (e.g., a “raster scan” of applications). Prescribed spot positions for a scanned proton beam delivery are typically arranged on a fixed (raster) pattern for each energy and therefore deliverable on a fixed scanning path within an energy layer (for example on a meander like path). By superposition of several proton beams of different energies, a Bragg peak can be spread out to cover target volumes by a uniform, prescribed dose. This enables proton therapy treatments to more precisely localize the radiation dosage relative to other types of external beam radiotherapy. During proton therapy treatment, a particle accelerator such as a cyclotron or synchrotron, is used to generate a beam of protons from, for example, an internal ion source located in the center of the cyclotron. The protons in the beam are accelerated (via a generated electric field), and the beam of accelerated protons is subsequently “extracted” and magnetically directed through a series of interconnecting tubes (called a beamline), often through multiple chambers, rooms, or even floors of a building, before finally being applied through an end section of beamline (called Nozzle) to a target volume in a treatment room.
As the volumes (e.g., organs, or regions of a body) targeted for radiation therapy are often below the surface of the skin and/or extend in three dimensions, and since proton therapy—like all radiation therapies—can be harmful to intervening tissue located in a subject between the target area and the beam emitter, the precise calculation and application of correct dosage amounts and positions are critical to avoid exposing non target areas to the radiation beyond what is necessary. However, target volumes within a body can shift and move periodically and even subconsciously or involuntarily, due to its role in or proximity to a normal respiratory or cardiac cycle, for example. Unfortunately, this movement can cause an unintended application of a proton therapy beam to neighboring healthy tissues (and/or organs at risk) for proton beams initially planned to treat the target volume. Typically the total prescribed target dose for a radiation treatment is delivered in multiple equivalent weighted fractions (total target dose divided in multiple portions of equivalent dose).
As a solution to this issue, techniques have been developed that mitigate the deviation of actual versus prescribed dose distributions caused by target volumes moving during applications of proton beams. One such technique is rescanning for pencil beam spot scanning technique, in which the intended (target) dose to a voxel in the target volume is gained by repeated application of multiple portions of its prescribed target dose for a fraction (i.e. visiting the voxel several times in a sequence—called rescanning). Typically, each prescribed spot position in a depth of a target volume (characterized by a dedicated energy of proton beam) can be visited multiple times (have multiple re-scans) to ensure (to the extent possible) that the entirety of the target area is treated. Also, by increasing the number of rescans (dividing the dosage into smaller fractions), the potential harm from overexposing a point in or neighboring the target volume can be minimized. Multiple rescanning provides an improvement over traditional approaches, particularly for the center of a target area. However, some issues still arise from the movement of the target area that occurs simultaneously with a rescanned proton beam application on a rigid path with fixed timing parameters and dose rate variability between layers only. For example, parts of the dose amounts planned for the target volume can still be blurred or smeared in or around the perimeters of target areas.